1. Field of the Invention
This invention relates in general to giant magnetoresistive (GMR) and magnetic tunnel junction (MTJ) sensors for reading information signals from a magnetic medium and, in particular, to GMR and MTJ sensors having improved corrosion resistive properties.
2. Description of the Related Art
Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive (MR) read sensors, commonly referred to as MR sensors, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater track and linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an xe2x80x9cMR elementxe2x80x9d) as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization in the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material (e.g., Nixe2x80x94Fe) separated by a layer of non-magnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors manifesting the SV effect.
FIG. 1 shows a prior art SV sensor 100 comprising end regions 104 and 106 separated by a central region 102. A first ferromagnetic layer, referred to as a pinned layer 120, has its magnetization typically fixed (pinned) by exchange coupling with an antiferromagnetic (AFM) layer 125. The magnetization of a second ferromagnetic layer, referred to as a free layer 110, is not fixed and is free to rotate in response to the magnetic field from the recorded magnetic medium (the signal field). The free layer 110 is separated from the pinned layer 120 by a non-magnetic, electrically conducting spacer layer 115. Hard bias layers 130 and 135 formed in the end regions 104 and 106, respectively, provide longitudinal bias for the free layer 110. Leads 140 and 145 formed on hard bias layers 130 and 135, respectively, provide electrical connections for sensing the resistance of SV sensor 100. IBM""s U.S. Pat. No. 5,206,590 granted to Dieny et al., incorporated herein by reference, discloses a GMR sensor operating on the basis of the SV effect.
Another type of spin valve sensor is an antiparallel (AP) spin valve sensor. The AP-pinned valve sensor differs from the simple simple spin valve sensor in that an AP-pinned structure has multiple thin film layers instead of a single pinned layer. The AP-pinned structure has an antiparallel coupling (APC) layer sandwiched between first and second ferromagnetic pinned layers. The first pinned layer has its magnetization oriented in a first direction by exchange coupling to the antiferromagnetic pinning layer. The second pinned layer is immediately adjacent to the free layer and is antiparallel exchange coupled to the first pinned layer because of the minimal thickness (in the order of 8 xc3x85) of the APC layer between the first and second pinned layers. Accordingly, the magnetization of the second pinned layer is oriented in a second direction that is antiparallel to the direction of the magnetization of the first pinned layer.
The AP-pinned structure is preferred over the single pinned layer because the magnetizations of the first and second pinned layers of the AP-inned structure subtractively combine to provide a net magnetization that is less than the magnetization of the single pinned layer. The direction of the net magnetization is determined by the thicker of the first and second pinned layers. A reduced net magnetization equates to a reduced demagnetization field from the AP-pinned structure. Since the antiferromagnetic exchange coupling is inversely proportional to the net pinning magnetization, this increases exchange coupling between the first pinned layer and the antiferromagnetic pinning layer. The AP-pinned spin valve sensor is described in commonly assigned U.S. Pat. No. 5,465,185 to Heim and Parkin which is incorporated by reference herein.
Another type of magnetic device currently under development is a magnetic tunnel junction (MTJ) device. The MTJ device has potential applications as a memory cell and as a magnetic field sensor. The MTJ device comprises two ferromagnetic layers separated by a thin, electrically insulating, tunnel barrier layer. The tunnel barrier layer is sufficiently thin that quantum-mechanical tunneling of charge carriers occurs between the ferromagnetic layers. The tunneling process is electron spin dependent, which means that the tunneling current across the junction depends on the spin-dependent electronic properties of the ferromagnetic materials and is a function of the relative orientation of the magnetic moments, or magnetization directions, of the two ferromagnetic layers. In the MTJ sensor, one ferromagnetic layer has its magnetic moment fixed, or pinned, and the other ferromagnetic layer has its magnetic moment free to rotate in response to an external magnetic field from the recording medium (the signal field). When an electric potential is applied between the two ferromagnetic layers, the sensor resistance is a function of the tunneling current across the insulating layer between the ferromagnetic layers. Since the tunneling current that flows perpendicularly through the tunnel barrier layer depends on the relative magnetization directions of the two ferromagnetic layers, recorded data can be read from a magnetic medium because the signal field causes a change of direction of magnetization of the free layer, which in turn causes a change in resistance of the MTJ sensor and a corresponding change in the sensed current or voltage. IBM""s U.S. Pat. No. 5,650,958 granted to Gallagher et al., incorporated in its entirety herein by reference, discloses an MTJ sensor operating on the basis of the magnetic tunnel junction effect.
FIG. 2 shows a prior art MTJ sensor 200 comprising a first electrode 204, a second electrode 202, and a tunnel barrier layer 215. The first electrode 204 comprises a pinned layer (pinned ferromagnetic layer) 220, an antiferromagnetic (AFM) layer 230, and a seed layer 240. The magnetization of the pinned layer 220 fixed through exchange coupling with the AFM layer 230. The second electrode 202 comprises a free layer (free ferromagnetic layer) 210 and a cap layer 205. The free layer 210 is separated from the pinned layer 220 by a non-magnetic, electrically insulating tunnel barrier layer 215. In the absence of an external magnetic field, the free layer 210 has its magnetization oriented in the direction shown by arrow 212, that is, generally perpendicular to the magnetization direction of the pinned layer 220 shown by arrow 222 (tail of an arrow pointing into the plane of the paper). A first lead 260 and a second lead 265 formed in contact with first electrode 204 and second electrode 202, respectively, provide electrical connections for the flow of sensing current Is from a current source 270 to the MTJ sensor 200. A signal detector 280, typically including a recording channel such as a partial-response maximum-likelihood (PRML) channel, connected to the first and second leads 260 and 265 senses the change in resistance due to changes induced in the free layer 210 by the external magnetic field.
Dual SV or MTJ sensors can provide increased magnetoresistive response to a signal field due to the additive response of the dual sensors. IBM""s U.S. Pat. No. 5,287,238 granted to Baumgart et al. discloses a dual SV sensor. FIG. 3 shows a dual spin valve sensor 300 wherein the spin valve structure is doubled symmetrically with respect a ferromagnetic free layer 308. The structure of the dual spin valve sensor is AFM1/Pinned1/Spacer1/Free/Spacer2/Pinned2/AFM2 providing ferromagnetic first and second pinned layers 304 and 312 separated by nonmagnetic first and second spacer layers 306 and 310, respectively, from a ferromagnetic free layer 308 which allows utilization of the conduction electrons scattered in both directions from the intermediate free layer 308. The directions of magnetization 305 and 313 (tails of arrows pointing into the plane of the paper) of the first and second pinned layers 304 and 312 are fixed parallel to each other by adjacent first and second antiferromagnetic layers 302 and 314, respectively. The direction of magnetization 309 of the free layer 308 is set at an angle of about 90xc2x0 with respect to the magnetizations of the two pinned layers and is allowed to rotate freely in response to an applied magnetic field.
The design of GMR and MTJ sensors typically includes a stack of metallic layers deposited on top of each other. During the fabrication process of the magnetic heads, an edge of the sensor stack is lapped to form part of an air bearing surface (ABS) facing the magnetic disk surface on which magnetic data is stored. At the ABS the lapped edge of the stack of metallic layers is exposed to air, humidity and any contaminants present in the disk file enclosure. Oxidation and corrosion of the metallic layers at the ABS can cause significant degradation of the sensor performance and reliability.
Therefore, there is a need for GMR and MTJ sensors having improved corrosion resistive properties of the metallic layers without adversely affecting their GMR properties.
It is an object of the present invention to disclose GMR and MTJ sensors comprising magnetic layers having improved corrosion resistive properties.
It is another object of the present invention to disclose GMR and MTJ sensors comprising pinned layers formed of Coxe2x80x94Fexe2x80x94X, where X is niobium (Nb), hafnium (Hf) or a mixture of niobium and hafnium (NbHf), having improved corrosion resistive properties.
It is yet another object of the present invention to disclose GMR and MTJ sensors comprising free layers formed of Nixe2x80x94Fexe2x80x94Y, where Y is tantalum (Ta) or chromium (Cr), having improved corrosion resistive properties.
In accordance with the principles of the present invention, there are disclosed several embodiments of GMR and MTJ sensors including an antiparallel (AP)-pinned layer structure and a laminated free layer structure. The AP-pinned layer comprises a first ferromagnetic (FM1) layer, a second ferromagnetic layer (FM2) layer and an antiparallel coupling (APC) layer sandwiched between the FM1 and FM2 layers. The FM1 and FM2 layers are formed of (Coxe2x80x94Fe)axe2x80x94Xb, where X=Nb, Hf or NbHf, 90%xe2x89xa6axe2x89xa695%, 5%xe2x89xa6bxe2x89xa610% and a+b=100%. The addition of 5 to 10 atomic wt. % of Nb, Hf or NbHf into Coxe2x80x94Fe improves corrosion resistance by a well-known self-passivation effect of the these metals due to formation of a thin surface oxide which protects the metal surface from further degradation. Addition of these metals results in a reduced grain size of the deposited FM1 and FM2 layers leading to an increase of the electrical resistivity of the layers. This increase in layer resistivity reduces sense current shunting through the AP-pinned structure resulting in an increased deltaR/R for the GMR sensors.
The laminated free layer structure comprises a first sublayer and a second sublayer. The first sublayer is formed of (Coxe2x80x94Fe)axe2x80x94Xb, where X=Nb, Hf or NbHf, 90%xe2x89xa6axe2x89xa695%, 5%xe2x89xa6bxe2x89xa610% and a+b=100%. The second sublayer is formed of (Nixe2x80x94Fe)axe2x80x94Yb, where Y=Ta or Cr, 90%xe2x89xa6axe2x89xa695%, 5%xe2x89xa6bxe2x89xa610% and a+b=100%. The addition of 5 to 10 atomic wt. % of Nb, Hf or NbHf into Coxe2x80x94Fe and 5 to 10 atomic wt. % of Ta or Cr into Nixe2x80x94Fe also improves the corrosion resistance and increases the electrical resistivity of the first and second sublayers, respectively, of the freelayer structure of the sensors.
The above as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed description.